the domain name isn't going to affect your Google ranking.
In general it's better to avoid underscores in domain names because some people won't realise they're there when the name appears in a hyperlink, or an email address, and that can lead to confusion.
It's also not a great idea to publish your thoughts about names in a web forum. I won't give a full explanation of the reason, but I'm sure you'll realise if you think about it - the best thing is to register your name first, then you can tell anyone you like.
Keep the name as short as possible. It's easier for people to remember.
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